In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, an image is generally formed on a surface of a sheet such as a paper sheet or a plastic film through the following process steps.
First, a surface of a photoreceptor body having photoelectric conductivity is evenly electrically charged and, in this state, exposed to light, whereby an electrostatic latent image corresponding to an image to be formed on the sheet is formed on the surface of the photoreceptor body (charging step and exposing step).
Then, toner (minute color particles) preliminarily electrically charged at a predetermined potential is brought into contact with the surface of the photoreceptor body. Thus, the toner selectively adheres to the surface of the photoreceptor body according to the potential pattern of the electrostatic latent image, whereby the electrostatic latent image is developed into a toner image (developing step).
Subsequently, the toner image is transferred onto the surface of the sheet (transfer step), and fixed to the surface of the sheet (fixing step). Thus, the image is formed on the surface of the sheet.
Further, a part of the toner remaining on the surface of the photoreceptor body after the transfer of the toner image is removed, for example by a cleaning blade or the like (cleaning step). Thus, the photoreceptor body is ready for the next image formation.
In the developing step out of the aforementioned process steps, a developing roller is used for developing the electrostatic latent image formed on the surface of the photoreceptor body into the toner image.
A known developing roller is produced, for example, by preparing an ion conductive rubber composition containing an ion conductive rubber such as an epichlorohydrin rubber as a rubber component, forming the rubber composition into a tubular body, and crosslinking the rubber component of the tubular body (Patent Document 1 and the like).
However, the electrical conductivity of the ion conductive developing roller is highly environment-dependent. Particularly, the ion conductive developing roller has significantly different resistances in a higher temperature and higher humidity environment and in a lower temperature and lower humidity environment. Therefore, the ion conductive developing roller is liable to cause imaging failure due to a difference in environment.
Since the ion conductive rubber such as the epichlorohydrin rubber is expensive, the cost reduction of the developing roller is difficult.
For the cost reduction and the suppression of the environmental dependence, it is contemplated to use an electron conductive agent such as electrically conductive carbon black in combination with a reduced proportion of the ion conductive rubber, or to use only the electron conductive agent without the use of the ion conductive rubber to impart the developing roller with electron conductivity.
If the electron conductive agent such as the electrically conductive carbon black is added to the rubber composition, however, the developing roller is liable to become less flexible to have a higher hardness. This may cause additional problems.
More specifically, the developing roller is liable to degrade the toner to reduce imaging durability, or is liable to have a reduced nip width when being in press contact with the surface of the photoreceptor body. Therefore, a formed image is liable to have a lower image quality.
The term “imaging durability” is defined as an index that indicates how long the image formation quality can be properly maintained when the same toner is repeatedly used for the image formation.
A very small part of toner contained in a developing section of the image forming apparatus is used in each image forming cycle, and the remaining major part of the toner is repeatedly circulated in the developing section. Since the developing roller is provided in the developing section and repeatedly brought into contact with the toner, whether or not the developing roller can prevent damage to the toner is a key factor to the improvement of the imaging durability. If the imaging durability is reduced, the formed image is liable to have white streaks in its black solid portion or have fogging in its marginal portion, thereby having a lower image quality.
To cope with this, it is contemplated to add a softening agent such as an oil or a plasticizer, or to use a liquid rubber such as a liquid nitrile rubber in combination with other rubber as the rubber component (Patent Document 2) to improve the flexibility of the developing roller.
However, the use of the softening agent and the liquid rubber is liable to increase the compression set of the developing roller. The developing roller having a greater compression set is liable to suffer from so-called permanent compressive deformation. That is, when the developing roller is kept in press contact with the photoreceptor body during the stop of the image forming apparatus and then is rotated to be brought out of the press contact, for example, the press contact portion of the developing roller is not restored to its original state. This may result in imaging failure such as uneven image.
Particularly, when the developing roller is incorporated in a developing unit of the image forming apparatus and kept in contact with the surface of the photoreceptor body for a longer period of time in the higher temperature and higher humidity environment in a storage test, for example, the softening agent is liable to bleed on the developing roller. The bleeding softening agent is liable to contaminate the photoreceptor body to cause imaging failure (e.g., a contamination line occurs in a formed image).
The liquid rubber is less liable to contaminate the photoreceptor body due to bleeding thereof, because the liquid rubber is crosslinked with the other rubber of the rubber component. However, an uncrosslinked lower molecular weight component and an oil component contained in the liquid rubber is liable to bleed. This may cause the contamination.